Re: portable way to get highest bit set?

Kaz Kylheku <[email protected]> Sun, 15 Oct 2023 05:26:44 -0000 (UTC)

Newsgroups	comp.lang.c,alt.comp.lang.c
Organization	A noiseless patient Spider
Message-ID	<[email protected]>

On 2023-10-11, Kaz Kylheku <[email protected]> wrote:
> E.g. 32 bit code:
>
>    uint32_t fill_mask_down(uint32_t x)
>    {
>      x |= x >> 1;    // e.g.   1000...0000 -> 1100...0000
>      x |= x >> 2;    // e.g.   1100...0000 -> 1111...0000
>      x |= x >> 4;    // e.g.   11110000...  -> 11111111...
>      x |= x >> 8;
>      x |= x >> 16;
>
>      return x;
>    }
>
> Thus:
>
>   uint32_t isolate_highest_bit(uint32_t x)
>   {
>      uint32_t m = fill_mask_down(x);
>      return m ^ (m >> 1);
>   }
>

I guess this went over people's heads?

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